1. Field of the Invention
This invention generally relates to IEEE 802.11 communications and, more particularly to a system and method for establishing a Burst Acknowledgement protocol between selected 802.11 peer stations.
2. Description of the Related Art
As noted in “A Short Tutorial on Wireless LANs and IEEE 802.11 by Lough, Blankenship and Krizman (computer.org/students/looking/summer97/ieee802), the IEEE 802.11 standard places specifications on the parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which actually handles the transmission of data between nodes, can use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position nodulation. IEEE 802.11 makes provisions for data rates from 1 Mbps to 54 Mbps, and calls for operation in the 2.4-2.4835 GHz frequency band (in the case of spread-spectrum transmission), which is an unlicensed band for industrial, scientific, and medical (ISM) applications. IEEE 802.11 also makes provision for data rates from 6 Mbps to 54 Mbps, and calls for operation in the 5.2 and 5.8 U-NII (Unlicensed Information Infrastructure) band.
The MAC layer is a set of protocols that is responsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random “backoff factor” which determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is clear, the transmitting node decrements its backoff counter. When the channel is busy it does not decrement its backoff counter. When the backoff counter reaches zero, the node transmits the packet. Since the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet, cannot be used for the radio frequency transmissions of IEEE 802.11. The reason for this is that when a node is transmitting it cannot hear any other node in the system which may be transmitting, since its own signal will drown out any others arriving at the node.
Whenever a packet is to be transmitted, the transmitting node first sends out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received successfully, as determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back-and-forth exchange is necessary to avoid the “hidden node” problem. In the hidden-node situation node A can communicate with node B, and node B can communicate with node C, however, node A cannot communicate node C. Thus, for instance, although node A may sense the channel to be clear, node C may in fact be transmitting to node B. The protocol described above alerts node A that node B is busy, and hence it must wait before transmitting its packet.
The above-mentioned “Stop and Wait” protocol, that uses of an ACK packet to minimize the “hidden node” problem, comes with a corresponding expense. For example, if a communication is not received, the transmitter will not repeat the communication until it receives a request. This request is an overhead cost that degrades the channel traffic bandwidth. That is, the throughput is too low. Further, protocol may be unable to support higher quality of service (higher bandwidth) communications in some scenarios.
Burst, or Block Acknowledgement is a mechanism defined by IEEE 802.11 Tge which allows for a form of selective-retransmission between wireless LANs which conventionally use the “Stop and Wait” protocol. While there is a mechanism to set up Burst ACK between two peers, the mechanism does not have a defined interface. There is no means established for an application to set up the Burst ACK mechanism with a peer. Further, even if the mechanism could be established, there is no means for establishing the mechanism for a selected class of service.
It would be advantageous if IEEE 802.11 communications could be enabled to selectively engage a Burst Acknowledgment protocol, to minimize system overhead.